The present invention relates to a multiplex transmission method based on the "non-destructive arbitrative CSMA/CD (Carrier Sense Multiple Access with Collision Detection) transmission system" and to a synchronizing method for multiplex transmission.
In recent years, with the rapidly growing car electronics, increased units and larger harnesses are becoming problems. As automotive multiplex transmission methods which can solve the aforementioned problems with the transmission systems and which provides higher reliability and versatility, various distributed control type LANs (Local Area Networks) have been proposed. Among them, a non-destructive arbitrative type CSMA/CD has attracted the industry's attention and it is extensively used because of its satisfactory open system (for instance, even if a node is added or deleted due to variations such as different grades or different destinations in the same car model, it is not necessary to change the remaining nodes), responsiveness and versatility.
In the non-destructive arbitrative CSMA/CD transmission system, each multiplex node starts the transmission of data frame by frame when it confirms that the use of a multiplex transmission line (multiplex bus) consisting of twisted paired wires or the like has already ended and the transmission line has stayed in an idle state for longer than a predetermined length of time, as shown in FIG. 1 (b). Then, the multiplex node compares its own transmission waveform with that on the multiplex transmission line for each bit, and if the two waveforms disagree, then the multiplex node determines that it is a signal collision and interrupts the transmission, then goes back to verify the idle state. A multiplex node, that is transmitting a signal with a higher priority level, finds that the waveform on the multiplex bus is the same as the signal of its own node; therefore, it does not detect any collisions and it continues transmission.
When the aforementioned transmission method is applied to data transmission in a motorcar wherein the transmission distance is short and the transmission speed is relatively slow, all multiplex nodes can observe a signal on the multiplex bus at the same time, permitting improved efficiency of use of the line. This transmission method is also advantageous in that it does not require a complicated control procedure such as random back-off.
As described above, according to the non-destructive arbitrative type CSMA/CD system, if messages are sent out from a plurality of multiplex nodes to the multiplex bus common to these nodes, priority-based control is performed. For instance, as shown in FIG. 2, when a message A with a higher priority level (see FIG. 2 (a)) competes with a message B with a lower priority level (see FIG. 2 (a)) in transmission on the multiplex bus, the priority control is carried out so that the message with the higher priority level is transmitted (see FIG. 2 (c)).
In an actual system, however, the length of a bit of a reference clock may change due to variations in the device of a clock generating source or other causes. For example, if a message A with a higher priority level sent from a multiplex node with a reference clock generated later and a message B with a lower priority level sent from a multiplex node with a reference clock generated earlier are placed in a transmission wait state as shown in FIG. 3, and if the reference clocks of the two multiplex nodes are different in speed, then a difference Tc (=Ta-Tb) occurs in the time required for the two nodes to check the idle state (see FIG. 3 (b) and (c)). The symbols Ta and Tb (Ta&gt;Tb) indicate the times required for the two multiplex nodes, which are related to the message A and the message B, respectively, to check the idle state of the multiplex bus from the moment the use of the multiplex bus (see FIG. 3 (a)) is finished.
Then, when the time Tb elapses from the moment the use of the multiplex bus is finished and one of the multiplex nodes verifies the idle state of the multiplex bus, causing the message B to be placed in the transmission state, the transmission of the message A is stopped by a carrier detecting function despite the fact that the priority level of the message A is higher than that of the message B. This means that the conventional multiplex transmission method has a problem that priority levels are ignored if there are variations in the bit length of the reference clocks of the multiplex nodes.
Further, the regular message format of a frame in the non-destructive arbitrative type CSMA/CD system consists of SOM (Start Of Message) which indicates the start of a message, data ID which indicates the contents of the data that follows, a data area of a specified length, an error check code such as CRC, an ACK signal area for causing all multiplex nodes to return receipt acknowledgment signal (ACK signal) on a bit basis, and EOM (End Of Message) which indicates the end of data, as shown in FIG. 1 (a). And when the message of the aforementioned format is transmitted, a receiving multiplex node first detects the rise of SOM of the message transmitted onto the multiplex bus to perform frame synchronization at the rise of SOM, and then it performs bit re-synchronization at the rising edge of the data.
If, however, SOM of the message consists of a special code comprising six bits in a dominant state (hereinafter referred to as dominant bits) as shown in FIG. 1 (b), and if a dominant bit follows immediately after the aforementioned SOM, then it takes more time to complete re-synchronization. More specifically, re-synchronization cannot be carried out as long as a dominant bit appears in succession in data; it cannot be carried out until a bit in a passive state (hereinafter referred to as a passive bit) and a dominant bit appear in succession following consecutive dominant bits.
Moreover, if there is a tolerance among multiplex nodes due to differences in the reference clock bit length, clock errors due to different clock speeds accumulate over the period from the moment the synchronization is made at the rise of SOM to the moment the re-synchronization is made at the rise of data.
Thus, the conventional synchronizing method in multiplex transmission has a problem in which bit synchronization cannot be performed and a bit error results if an accumulated clock error increases due to prolonged re-synchronization.